What Is Depression?

Depression is one of the most common mental illnesses in the United States.

Depression is a serious mental illness in which feelings of sadness and loss of interest last for weeks and interfere with daily life.

All people experience moments when they feel sad or blue, but these feelings usually pass within a couple of days and are not indicative of depression.

Depression can cause deep emotional pain both to the person experiencing it and, often, to that person's close family and friends.

Types of Depression

There are several different types of medically recognized depression.

The most common type of depression is called major depression, and it occurs when your symptoms interfere with your enjoyment of life or daily functions — including your work, sleep, and eating habits — for at least two weeks straight.

Some people experience only one episode of major depression in their life, while others may go through numerous episodes of the illness.

In comparison, people with another condition known as persistent depressive disorder — also known as dysthymia — experience less severe mood symptoms that last continuously for at least two years.

During this time, there may also be periods when the person experiences major depression.

Other common types of depression include:

Postpartum depression, in which mothers experience symptoms of major depression after giving birth (mood impairment is much stronger, and lasts longer, than the "baby blues" that many new mothers experience)

Seasonal affective disorder (SAD), in which depression sets in during winter (and sometimes fall) and is associated with a lack of sunlight

Psychotic depression, in which severe depression is paired with some form of psychosis, such as delusions and hallucinations

Premenstrual dysphoric disorder, in which symptoms of depression develop a week before a woman's period and pass after menstruation

Some people who experience depression may have bipolar disorder — formerly called manic-depression illness — which is characterized by moods that cycle between extreme highs (mania) and lows (depression).

Depression Prevalence

In the United States, depression is one of the most common mental health disorders.

In 2014, about 15.7 million adults ages 18 and above — or 6.7 percent of adults — had at least one major depressive episode, according to the National Institutes of the Health (NIH).

Dysthymia, on the other hand, affects about 1.5 percent of the adult population.

Worldwide, depression affects some 350 million people, according to the World Health Organization.

Different cultures experience different rates of depression.

For instance, the prevalence of major depression among all ages appears to be low in Japan (2.2 percent) and high in Brazil (10.4 percent), according to a 2010 report in the journal Annual Review of Public Health.

Having one depressive episode increases your risk of having another later in life.

In fact, 50 percent of people who recover from their first episode of depression go on to have one or more additional episodes in their life, while 80 percent of people who have had two episodes experience a recurrence, according to 2007 report in Clinical Psychology Review.

Causes and Risk Factors

There are numerous factors that can trigger the onset of depression, including bereavement, illness (such as cancer or chronic pain), social isolation or loneliness, and stressful life events (such as divorce or money problems).

But scientists don't know exactly why some people develop depression and others avoid it. Several factors most likely contribute to the development of depression, including:

Genetics (mood disorders and suicide run in families)

Trauma or abuse at an early age, which can cause long-term changes in how the brain deals with fear and stress

Brain structure and chemistry

Substance abuse

Hormonal changes, such as from pregnancy or thyroid problems

Women are 70 percent more likely to experience depression than men, and non-Hispanic blacks are 40 percent less likely to experience it than non-Hispanic whites, according to the NIH.

In addition, people ages 18 to 25 are 60 percent more likely to experience depression than people ages 50 and above.

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